1. Field of the Invention
This invention relates generally to the precise measurement of atomic vapor density in the fabrication of materials and devices during deposition, and particularly to the control of deposition rate and composition using atomic absorption spectroscopy.
2. Brief Description of the Prior Art
The fabrication of certain materials and devices, such as semi-conductor lasers, requires the control of very low growth rates, often less than 0.1 nm/s with an accuracy of greater than 1%.
Quartz Crystal Microbalances are commonly used to determine the amount of deposited mass. However such devices cannot distinguish among different materials, and are sensitive to other factors, such as temperature and stress.
Electron Impact Emission Spectroscopy has been used to determine vapor flux density and thus deposition rates. Although this procedure can distinguish among different materials and is highly sensitive, its long-term stability and operational pressure range limits or excludes its use in some processes. For example, its limitation to high vacuum conditions (less than 10.sup.-4 mbar) prohibits its use in thin film fabrication processes such as sputtering and reactive evaporation.
Mass Spectroscopy methods are both material specific and highly sensitive, but are subject to the same limitations as Electron Impact Emission Spectroscopy.
Another drawback to the prior art methods and devices described above is that all require the presence of a sensor within the processing chamber. The location of a sensor within the processing chamber may disturb the process or contaminate the chamber. Additionally, sensors generally must be physically located in a different portion of the chamber than the location of the substrates where the actual film growth occurs. The difference between the substrate location and the sensor location may introduce errors in process control.
Atomic Absorption Spectroscopy is a promising technique for measuring vapor density. This technique is material specific, highly sensitive, and non-intrusive. It has been widely accepted as a standard tool for chemical analysis of elements. Atomic Absorption Spectroscopic devices, employ an optical system consisting of a special light source, a wavelength selection device and a photodetector. U.S. Pat. No. 3,654,109 discloses the basic concepts of such systems. Notwithstanding, the favorable characteristics of such devices, the photodetector has its maximum output at zero deposition rate, i.e., without the presence of vapor flux. This reference baseline at zero deposition rate can be affected by changes in the light source intensity, the optical system alignment and the transmission of optical windows. The signal changes brought about from these undesirable effects cannot be separated from the real absorption signal after the deposition process starts. The inability to separate these signal changes causes baseline instability and can significantly affect the accuracy and repeatability of vapor flux measurements.